Vitamin a ester compositions



State 3 Claims. or. 260-410) This invention is concerned with certainnovel composi tions containing higher fatty acid esters of vitamin A.

Esters of vitamin A, that is esters of vitamin A alcohol with alkanoicor alkenoic acids having from about ten upto about twenty-two carbonatoms in the chain, are especially useful forms of the vitamin.Particularly the longer chain compounds have high solubility in fats,are ex cellently stable on storage, and are readily incorporated invarious pharmaceuticals, human foods, or animal foods. For these reasonsthey are preferred to vitamin A alcohols for theraupetic and othercommercial use.

In one method for the manufacture of synthetic vitamin A, the acetate,or if preferred, another lower alkanoic acid ester, is obtained as anend product. This type of compound has been converted to the desirablelong-chain fatty acid esters by saponification and re-esterification,for example, with a long-chain fatty acid chloride. Such a process has anumber of deficiencies. The extra steps of saponification and isolationof the vitamin A alcohol are wasteful, involving decomposition andlosses due to oxidation of the vitamin. The use of the long-chain fattyacid chlorides is unsatisfactory, since they are highly corrosivecompounds and are unstable, readily hydrolyzing in the presence ofmoisture with the evolution of hydrogen chloride. A process which wouldavoid these and other difliculties of known procedures would be ofconsiderable value.

This invention at last provides such a process. This application is adivision of the application Serial No. 681,182, filed August 30, 1957,and now U.S. Patent 2,971,966 which application Was in turn acontinuationin-part of Serial No. 443,970, filed July 16, 1954, and nowabandoned. That application was in turn based on application, Serial No.269,578, filed on February 1, 1952, by William E. Stieg et al., now US.Patent 2,693,435. Broadly speaking, the present novel method involvestransesterifying relatively simple vitamin A esters with longer chainaliphatic non-vitamin esters. The transesterification is achieved bycontacting a lower alkanoic acid ester of the vitamin with an aliphaticester of a longer chain alkanoic or alkenoic acid in the presence of analkaline catalyst, preferably under substantially anhydrous conditions.More complex mono-esters of fatty acids and the like are thus obtained,and by a process which is more direct and simpler than the conventionalmethod.

In one preferred embodiment of this invention, a lower alkanoic acidester of vitamin A, e.g. one where the acid chain contains, say, betweentwo and six carbon atoms, is reacted with a lower ester of an aliphatichydrocarbon carboxylic acid having from about ten to twenty-two carbonatoms with which it is desired to form a new vitamin A ester. Theselonger-chain esters generally are first formed from the lower one tofour or five carbon monohydric aliphatic alcohols and the highermonobasic alkanoic or alkenoic acid. The reaction is conducted at amildly elevated temperature to accelerate the process, undersubstantially anhydrous conditions to minimize undesirable sidereactions, and in the presence of an alkaline transesterificationcatalyst, particularly an alkali metal or alkaline earth metal compound.The desired vitamin A fatty acid ester is recovered as the product andthe corre- Unit H catalyst throughout the reaction mass.

spondin g lower aliphatic alcohol-lower alkanoic acid ester is obtainedas the principal by-product.

By heating to a suitable temperature or by lowering the pressure or byboth practices, the lowest boiling component of the reaction mass (e.g.the by-product ester of the lower alcohol) is distilled out of themixture, preferably as it is formed, thus assisting in completing thedesired reaction. The elevation of temperature of the mix ture alsoassists in forming a completely liquefied system which is homogeneousand more readily stirred. Some of the lower aliphatic alcohol esters ofthe long-chain aliphatic acids are solid at room temperature. A range oftemperature from about 20 C. to about C. is most suitable for thereaction. Removal of the by-product ester has a very distinct advantagein resulting in the formation of vitamin A esters of very high purity.In fact, these materials are often of such high purity that they readilycrystallize. In any event the purity of the resulting ester isinvariably in excess of 80%, and generally of 95-l00%. They also haveother advantages due to their high purity, such as a very low level oftaste and the absence of deleterious degradation products andby-products. This process has proven exceedingly useful for thepreparation, on a large scale, of vitamin A esters of very high purity,and with the minimum of loss due to decomposition. The product thusproduced is free of impurities which result from the previously usedprocess of saponification and direct esteriiicat-ion. Furthermore, thedanger of corrosive estrification agents is completely eliminated by thepresent process.

it has been found most important, in order to achievetransesterification at practical rates, to have present in the reactionmixture an alkaline catalyst, preferably an alkali metal or alkalineearth metal compound or the alkali metal itself. The compound chosen isdesirably in the form of an alkoxide, oxide, or hydroxide, for examplesodium hydroxide, potassium hydroxide, lithium hydroxide, bariumhydroxide, calcium hydroxide, calcium oxide, barium oxide, sodiummethoxide, lithium methoxide, potassium ethoxide, barium methoxide,calcium methoxide, magnesium isopropoxide and so forth. Of the alkalimetals, a particularly useful catalyst is an alloy of sodium andpotassium containing from 50% to 15% sodium and from 50% to potassiumwhich is liquid below 50 C. A 5050 sodium potassium alloy isparticularly suitable. The catalyst need only be used in a lowproportion, e.g. 0.20 mole per mole of vitamin A reactant or less; andsubstantially between 0.01 and 0.1 mole per mole of the vitamin ester isgenerally enough. It is important that good contact of the catalyst andreactants be obtained, and efiicient mechanical agitation is normallymaintained for this purpose. The catalyst may be added as a solution oras a suspension in a suitable solvent such as a lower alcohol. Thisassists in improving distribution of the If a solvent is utilized forthe addition of the catalyst, this, too, is removed during thedistillation of the ester by-product.

Approximately equimolecular proportions of the longchain fatty acidester and the shorter chain vitamin A ester starting materials aresuflicient for the usual reaction. An excess of one or the otherconstituent can readily be present without harm; however, althoughlittle value results therefrom. Either or both reactants may be in crudeor purified form, depending on the product desired. A notably usefulsimple vitamin A ester is the acetate, which is most readily availablein commerce. However, vitamin A propionate, butyrate, isobutyrate,v-alerate, etc., can also be transesterified by this new process.Valuable reactants among the longer chain fatty acid esters are thepalmitates, laurates, myristates, stearates, esters of unsaturated fattyacides of either cis or trans configuration, such as oleates,linoleates, elaidates, erucates, brassidates,

undecylenates, and the like, estrified with monohydric aliphaticalcohols, e.g., methanol, ethanol, methoxy-ethanol, ethoxy-ethanol,allyl alcohol, propanols, butanols, etc.

In carrying out the process of this invention, the longchain ester isgenerally heated in a suitable vessel and, in the case of those esters,which are solids, first melted. The vitamin A lower alkanoic acid ester,e.g. the acetate, which also may be solid in purified form, is mixedwith the other reactant and the catalytic metallic compound isintroduced. The mixture is stirred well. It is preferable to dry thereactants before addition of the catalyst, so that the desirableanhydrous condition is maintained. Good agitation is necessary to assuresufficient contact of the catalyst and reactants, particularly sincecertain of the catalysts may have low solubility in the reactionmixtures. The temperature is gradually elevated. Vacuum may be appliedto the warm mixture, so that the ester formed from the lower aliphaticalcohol and lower alkanoic acid is distilled off and completion of thereaction accelerated. ln'fact, application of vacuum is preferable wherethe ester by-produot is high-boiling. This by-product can be collected,and when approximately an equimolar proportion thereof has beenobtained, the reaction is considered complete. If a vacuum is used forthe removal of the ester, some losses may occur in the vacuum systemunless an efficient trap is utilized. However, experience with theprocess will-indicate how long any particular esterification takes tocomplete and the volume of by-pro'duct ester collected need no longer beused as an index of completion of the reaction. Transesterification maytake from one hour to several hours, depending somewhat on thetemperature, the catalyst, and the equipment used. In general it is bestto heat the reaction mixture above about 20 C. but not much above 80 C.Little or no destruction of the active compound occurs even at 80 C. orsomewhat more, and high yields of vitamin A long-chain fatty acid estersare still obtained. Although a high boiling organic solventlike anaromatic hydrocarbon (which must, of course, have a boiling point higherthan the by-product ester to be distilledout of the reaction mixture)may be used, this is not the preferred procedure, since it is notessential for high yields and it may increase the difliculty ofrecovering the compound.

After the reaction has been substantially completed, the product may bedissolved in a suitable solvent, that is, one capable of dissolving thevitamin A long-chain fatty acid ester but'yet of a fairly low boilingpoint for ease of subsequent removal. Examples of useful solvents arebenzene, petroleum ether, chloroform, diethyl ether, methylenedichloride, and so forth. The catalyst may then be removed by washingthe organic solvent solution with water or with a dilute acid in justsuflicient amount to neutralize the alkaline compound. Various weakacids are particularly efiective for this purpose. In fact, a solutionof carbon dioxide in water is quite satisfactory. Weak organic acidssuch as acetic acid may also be used. The organic solution may then bedried and the solvent removed to yield the desired long-chain fatty acidester of vitamin A. It is obvious that toxic material should not be leftin-the final reaction mixture or product, if the compound is intendedfor use in nutrition or in therapy.

The products prepared by the present process, that is, long chainaliphatic acid esters of vitamin A of high purity possess considerablestability due to the absence of deleterious by-products. However, forcertain uses it is advisable to add to the vitamin certain antioxidants.A substantial number of such stabilizing agents have been testedwith'little or no'success, but it has been found that alkylated phenolicor alkylated polyhydric phenolic food-grade antioxidants aresurprisingly useful for this purpose. An unexpectedly high degree ofstabilization is obtained by the addition of sufficient of thesematerials, generally less than about 3%, to the products of thisinvention. The most suitable proportion of a given agent for a specificcomposition may be determined with the minimum of testing, usingwell-known methods of evaluation. In general, the most favorableproportion ranges from about 0.2% to 2.0% by weight based on the vitaminA alcohol content of the product, but certain materials may require moreor less than this. Stable vitamin A compositions of high potency arethus provided. Such compositions containing a major proportion, or more,of a vitamin A ester and up to about 23% of these antioxidants areespecially useful and convenient for bulk use of vitamin A in the foodand drug industries.

The alkyl group of the chosen alkylated phenolic or polyhydric phenolicfood-grade antioxidant has preferably at least about three carbon atomsand not more than about six, and the tertiary butyl group isparticularly useful for this purpose. When a polyhydric phenolderivative, such as a compound related to catechol or hydroquinone, isused, it may take the form of a mono-lower alkyl ether. Among the mostuseful stabilizing agents for the new compositions of this invention arebutylated hydroxyanisoles, 3-tertiary-butyl-4-hydroxyanisole,2-tertiary-butyl-4-hydroxyanisole,2,6-di-tertiary-butyl-4-methyl-phenol, 2,2methylenebis-(4-methyl-6-tertiary-butylphenol),tertiary-butyl-meta-cresol, 2,5-di-tertiary-butylhydroquinone,structurally related compounds and mixtures of these. It is, of course,necessary to use stabilizers of low toxicity for pharmaceuticalpreparations. The stabilizer'may be added to the novel oil-vitamin Acompositions after removal of the alkaline catalyst.

It should be noted that not only may a single long-chain aliphaticacid-short-chain aliphatic alcohol ester be utilized as one of thestarting materials in the present process, but mixtures of such esters,such as mixtures of methyl palrnitate and ethyl oleate, may also beused. Such mixtures have certain advantages in that the mixtures oflongchain aliphatic acid esters of vitamin A ordinarily are'liquid atroom temperature, whereas the esters with a single acid being of highpurity often tend to crystallize at room temperature. It is obvious thatthere are some advantages to liquid vitamin A ester mixtures of highpurity, since liquids may more readily be added -to mixedproducts andmay be measured out by volume more conveniently.

The following examples are given by way of illustration and are not tobe construed as the sole embodiments of this invention. It is to beunderstood that protection hereof is only to be limited by the specificwording of the appended claims.

Example I One-third of a mole (109.5 g.) of pure crystalline vitamin Aacetate was mixed with one-third of a mole (90.2 g. of methylpalrnitate. The mixture was placed in a 500 ml. round-bottomed flaskequipped with a capillary tube for introducing a fine stream of nitrogengas and an outlet above the surface of the liquid connected through aDry Ice-cooled trap to a high vacuum pump. The mixture was heated inthis flask at a temperature of 4555 C. to melt the solid material. Themelt was subjected to high vacuum for an hour to remove traces ofethanol and water, and then 4.0 g. of dry sodium methylate was added.The flask was again evacuated and the mixture was heated at 5560 C. forabout two hours. During this period over of the theoretical amount ofmethyl acetate by-product was collected in the Dry Ice-cooled trap. Thereaction was continued for another hour and the weight of the residue inthe flask was then checked. It was found that between 98 and 100% of thetheoretical loss in weight due to distillation of methyl acetate hadoccurred. The residual product consisted of practically pure vitamin Apalmitate containing the catalytic metallic alkoxide. This palmitate wasfreed of the sodium methylate by dissolving it in 2 to 4 volumes ofmethylene chloride and washing the solvent solution with Water. When thewater washings were found to be neutral, the solvent was removed byevaporation in vacuo at 40-50 C. The final vitamin A palmitate productWeighed 169 g.

and was shown to assay 98-100% pure by the USP. XIV spectrophotometricmethod. Its refractive index was n =1.555l.556, and the saponificationequivalent was found to be 520-530.

Example 11 One-third of a mole (109.5 g.) of pure crystalline vitamin Aacetate was mixed with one-third of a mole (90.2 grams) of pure methylpalmitate in apparatus as described in the example above. The mixturewas heated under vacuum as before for removal of traces of ethanol andwater. Then 0.35 gram of lithium methoxide dissolved in ml. of methanolwas added. The methyl acetate was distilled from the reaction mixture at55 60 C. over a period of 90 minutes. There was a loss of 24.5 grams,which showed 100% reaction. The residue was dissolved in three volumesof methylene chloride, treated with water saturated with carbon dioxide,and then washed three times with water. After drying the organic solventsolution over anhydrous sodium sulfate, the solvent was removed undervacuum. One hundred seventy grams of vitamin A palmitate assaying 99%pure were obtained.

Example III Vitamin A acetate and methyl palmitate were recated by theprocedure of Examples I and II with the exception that magnesiumethoxide was used to catalyze the ester interchange. The vitamin Apalmitate produced was of approximately the same purity and was obtainedin similar high yield.

Example IV One-third of a mole (109.5 grams) of pure vitamin A acetatewas mixed with one-third of a mole (94.8 grams) of pure ethyl palmitate.The reaction was carried out as before, using a catalyst consisting of0.2 gram of sodium metal dissolved in 10 mls. of anhydrous ethanol.Approximately two hours were required to complete the removal of theethyl acetate by-product. After working up the residue as in theprevious examples, a 170- gram yield of vitamin A palmitate assaying99l00% pure was obtained.

Example V Equimolecular proportions of vitamin A acetate and methylpalmitate were commingled and the mixture melted at 45"-50 C. in areaction chamber equipped with a stirrer, a thermometer, a connection toa Dry Ice-cooled trap attached to a vacuum pump, and a dropping funnelfor introduction of the catalyst. The melted mixture was stripped fortwo hours under high vacuum to remove volatile material, such as water.A solution of sodium methylate in methanol, having a concentration ofgrams per 100 ml. was used as a catalyst. The course of thetransesterification was followed by observing the rate of distillationof the methyl acetate and the loss of weight in the reaction products.The following table records the observations made.

The reaction vessel was found to have lost 98% of the weight calculatedto be lost by complete distillation of methyl acetate by-product. Thus,the reaction was 98% complete. The residual product was dissolved inthree volumes of hexane, washed once with water containing carbondioxide, and washed three times with pure water.

The hexane solution was then concentrated under vacuum and the vitamin Apalmitate product recovered. The weight of this showed a yield of 96%.The new ester assayed 100% as vitamin A palmitate by the U.S.P. XIVassay method.

Example VI One mole of pure crystalline vitamin A acetate (328 grams)and one mole of methyl palmitate (270 grams) were melted together in achamber similar to that described in Example V. To this mixture wasadded a solution of 4.5 grams of magnesium metal in 100 mls. of warmmethanol. The reaction mixture was heated to 55 C. and this catalyst wasadded with stirring. The reaction vessel was evacuated and the mixtureof methanol and methyl acetate was immersed in a Dry Ice bath. Thereaction was discontinued after two hours and it was found that thereaction mixture had lost 71 grams of weight (corrected for the weightof catalyst used). The methyl acetate-methanol mixture collected in thetrap was found to contain 71.5 grams of methyl acetate. Thus, thereaction was 95-96% complete. The product was dissolved in three volumesof hexane and, after washing with water containing carbon dioxide, itwas washed twice with pure water. The hexane was then removed undervacuum. The residue, light-colored vitamin A palmitate, weighed 520grams and assayed 98%.

Example VII Example VIII A mixture of 0.75 mole of vitamin A acetate and0.75 mole of ethyl myristate was treated with a small amount (about 0.05mole) of sodium methylate. The mixture was stirred and heated to atemperature of 45 -55 C. The ethyl acetate by-product was distilled outunder vacuum as in the previous examples. After recovery, the vitamin Amyristate was found to assay 99.5% pure. It had a refractive index of n=1.5632 and readily crystallized on standing in a refrigerator.

Example IX Equimolecular proportions of vitamin A butyrate and propyllaurate were mixed and about 0.1 mole of lithium hydroxide dissolved ina small volume of methanol was added. The mixture was heated at 50 C.under vacuum for several hours. The propyl butyr-ate which distilledfrom the reaction mixture was collected in a Dry Ice-cooled trap. Whenapproximately one molecular proportion had been collected, the reactionmixture was cooled, dissolved in a solvent and washed with watercontaining carbon dioxide to remove the alkaline catalyst. An almostquantitative yield of vitamin A laurate was obtained. The product wasmixed with 0.5% of commercial butylated hydroxyanisole and it was thendried under vacuum using a stream of dry nitnogen.

Example X One mole (328 g.) of vitamin A acetate and one mole (296.4 g.)of methyl oleate were melted together in a 3 l. round-bottomed flaskequipped with a capillary inlet tube for the introduction of nitrogengas and an outlet above the surface of the liquid connected through acold trap (solid carbon dioxide) to a high vacuum pump. The mixture wasthoroughly dried by subjecting same to a high vacuum at a temperature of4550 C. for one hour. A solution consisting of 7 g. of barium oxide in70 ml. of methanol was then added and the methanol and by-product methylacetate were both distilled in vacuo in the preceding fashion. Thereaction was complete within two hours. The catalyst was thenneutralized by washing first with water saturated with carbon dioxide,followed by pure water until successive water washes has neutral pHs.The organic layer was then collected and thoroughly dried in vacuoproviding vitamin A oleate, 545 g. assaying 95.4% pure according to theU.S.P. XIV assay, refractive index n =1.5484. The product was mixed with1.0% of commercial butylated hydroxytoluene.

Example XI One-third 'of a mole each of vitamin A isobutyrate and methylpalmitate were mixed with ml. of a solution of sodium hydroxide inmethanol. The mixture was stirred under vacuum and heated to atemperature of -70 C. for one hour, at which time the reaction mixtureexhibited a refractive index of n =1.5372. A second 10 ml. portion ofcatalyst was then added and the mixture once again heated under vacuumto a temperature of -70 C. for a period of one hour, at-which time therefractive index of the reaction mixture became n =1.5448. Lastly, a 5ml. portion of catalyst was added and the reaction continued for anadditional hour, the final refractive'index value being -n =1.5460. Theproduct was recovered by first neutralizing the catalyst, followed bywashing and drying in the usual manner. A yield of 79% of vitamin Apalmitate was obtained, which assayed for 1,490,000 'U.S.P. units ofvitamin A activity per gram of compound.

Example XII total of six hrs., the reaction mixture was cooled and thevitamin A palmitate recovered in the manner just de scribed in the:preceding examples =(n =l.5470).

Example XIII The process of Example X'is repeated substituting 4.8 g. ofcalcium hydroxide for the barium hydroxide specified with the recoveryof vitamin A oleate as described.

Example XIV The process of Example X is repeated substituting 201 g.

.o .3 of methyl hendec-lO-enate for the methyl oleate specified with therecovery of vitamin A hendec-lO-enate instead of vitamin A oleate.

Example XV The process of Example [is repeated substituting onethird ofa mole of vitamin A valerate for the vitamin A acetate employed. Asomewhat higher temperature, ap proximately -80", is employed after theaddition of the catalyst, due to the higher boiling point of thelay-product methyl valerate. Vitamin A palmitate in excess of 80% purityis obtained.

Example XVI The process of Example I is repeated substituting onethirdof a mole of allyl palmitate for the methyl palmitate employed in thatprocess. Vitamin A palmitate is recovered in a similar fashion althougha somewhat longer time is required for the calculated quantity of allylacetate to distill.

Example XVII The process of Example I is repeated substituting methylarachidate and vitamin A caproate for the reactants specified. Themethyl caproate is distilled from the reaction mixture in vacuo and thevitamin A arachidate recovered in the usual fashion.

Example XVIII Crystalline vitamin A acetate, 0.33 mole, and methylpalmitate, 0.33 mole, are mixed and 1 ml. of sodium potassium alloy(SO-50) is added. The mixture is heated at about 50 'C. in vacuo and thelay-product methyl acetate distilled. The catalyst is neutralized andthe product recovered as described in Example I. A yield of 172 g. (97%)of vitamin A palmitate assaying 97% pure is obtained, n =l.5563.

What is claimed is:

1. A stabilized vitamin A composition consisting of a vitamin A esterofan aliphatic hydrocarbon carboxylic acid having from about ten toabout twenty-two carbon atoms and from 0.2% to 2.0% by weight of afoodgrade antioxidant selected from the group consisting of alkylatedphenols and alkylated polyhydric phenols wherein the alkyl group hasbetween about 3 and 6 carbon atoms.

2. A composition was claimed in claim 1 wherein the antioxidant isbutylatedhydroxyanisole.

3. A composition as claimed in claim 1 wherein the antioxidant isbutylated hydroxytoluene.

No references cited.

1. A STABILIZED VITAMIN A COMPOSITION CONSISTING OF A VITAMIN A ESTER OFAN ALIPHATIC HYDROCARBON CARBOXYLIC ACID HAVING FROM ABOUT TEN TO ABOUTTWENTY-TWO CARBON ATOMS AND FROM 0.2% BY WEIGHT OF A FOODGRADEANTIOXIDANT SELECTED FROM THE GROUP CONSISTING OF ALKYLATED PHENOLS ANDALKYLATED POLYHYDRIC PHENOLS WHEREIN THEALKYL GROUP HAS BETWEEN ABOUT 3TO 6 CARBON ATOMS.